This invention relates to an automatic level control system for a vehicle having sprung and unsprung masses connected by shock absorber means and further including fluid power means activatable to level the sprung mass with respect to the unsprung mass by increasing or decreasing a fluid pressure in response to electrical signals. Such a control further includes sensor apparatus to sense the relative level of the sprung and unsprung masses within a predetermined trim band. When the vehicle is standing, sprung and unsprung masses generally maintain a stable relative level between vehicle loading events; and an automatic level control may be straightforward and simple in design. However, when the vehicle is moving over rough surfaces, the relative level of the sprung and unsprung masses is continually changing; and the design of a stable and effective automatic level control is not so straightforward.
Generally, in an automatic level control, it is desired to correct the relative level of sprung and unsprung masses following permanent changes in said level due to vehicle loading events but not to attempt correcting the level in response to oscillatory changes in said relative level caused by vehicle movement over rough surfaces or by transient vehicle accelerations. This is generally accomplished with a requirement that an out-of-trim condition be present for a certain predetermined time period to be recognized as such. However, the exact duration of that time period is subject to conflicting requirements under different vehicle operating conditions. For example, when the vehicle is cruising on the highway, a long period is desired to avoid reaction to long transients due to a curve in the road or other highway features or to vehicle accelerations. On the other hand, when the fluid power means is activated to correct the vehicle level, it is desirable to check for correct trim much more frequently in order to avoid overshoot in the correction. The time requirement upon initial vehicle operation is once again different: somewhere between the previous two in duration. In addition, certain accessories are included on a vehicle, the actuation of which provide a strong probability of a forthcoming vehicle load change which may require vehicle levelling action. Among these are passenger or luggage compartment doors and the vehicle ignition switch.